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COLORADO DEPARTMENT OF PUBLI~ALTH AND THE ENV/RONMEM; Wates~a[ity Control Division Rationale -Page S, Permit No. CO-00041467 Whole Effluent Toxicity (WET) Testinr: Outfalt 004A For this facility, chronic WET resting is required. (See Parts I.A and I.B of the permit.) a) Puroose of WET Testinu: The Water Quality Comrol Division has estabtished the use of WET testing as a method for identifying and controlling toxic discharges from wastewater treatment facilities. WET testing is being utilized as a means to ensure shat there are no discharges of pollutants "in amounts, concentrations or combinations which are harmful to [he beneficial uses nr toxic ro humans, animals, plants, or aquatic life" as required by Section 3.1.11 (1) of the Boric Standards and Methodolotties (or Surface Waters. b) Instream Waste Concentration (/WCI: Where monitoring or limitations for WET are deemed appropriate by the Division, chronic irtstre~vn dilution as represented by the chronic /WC is critical in determining whether acute or chronic conditions shall apply. For those discharges where the chronic l1VC is greater than (> ) 9.1 %, chronic conditions apply, where the IWC is less than or equal to {5) 9.1 acute conditions apply. The chronic IWC is determined using the following equation: IWC = [Facility Flow (FF)/(Stream Chronic Low Flow (annual) + FF)J X 100`90 The receiving stream flows toed to calculate the IWC rill be those for Woods Creek. The flows and corresponding IWC for the appropriate discharge paint are: LAscharge ;: ChroniaLow. T'atii(ity Design.: JWC; .(46):: Poin[ .:..Season Flow; .: :.... : :.... Flow, c s :. (f) :::: . , . . . 004A Oct-May 1.54 0.50 24.5 Jun-Jul 9.10 0.96 9.54 Aug-Sep 3.38 0.96 22.1 /n addition to the IWC value for determining whether chronic or acute WET testing requirements are applicable, the classification of the receiving stream must be considered. According to the Colorado Water OualiN Control Division Biomonitorine Guidance Document. dated July 1, 1993, chronic WET testing is required for facilities where the receiving stream has a Class 1 Aquatic Lije use or Class 2 Aquatic Lije use with all of the appropriate aquatic life numeric standards; otherwise, acute WET testing is required. Since this receiving water is classified as Class 2 Aquatic Lije use, with standards jilt metals and ammonia, the Division has determined that chronic WET testing is applicable. c) Chronic WET Limits: Whole Effluent Toxicity data for the previous two-year period were reviewed. These results are summarized below.